Tryptic hydrolysis of glycinin and its subunits

Glycinin, the major storage protein of soybeans, was modified by alkali (pH 12.0) and acid (pH 2.0) induced denaturation, and by unfolding in 6 M urea followed by cleavage of the disulphide (SS) bonds and blockage of the sulphydryl (SH) groups with iodoacetoamide to produce carboxyamidemethyl (CAM)‐glycinin. The CAM‐acidic and CAM‐basic subunits of the protein were also isolated. All the above samples were used to investigate their rate (pH‐stat method) and extent (gel filtration analysis) of hydrolysis by trypsin at pH 8.0, 25°C. Native and acid‐denatured glycinin and the CAM‐basic subunits were hydrolysed slowly over a period of several hours whereas the CAM‐acidic subunits, alkali‐denatured glycinin, and CAM‐glycinin were attacked at a very fast rate by the enzyme. The molecular weight distribution of the peptide fragments varied in all samples. Characteristic peaks of molecular weight above 30 000, around 7000 and below 5000 daltons were observed. In modifications where unfolding of the protein and SS reduction was implicated the peptide fragments were shifted toward the lowest peak(s) i.e. below 5000 daltons.